The manual transmissions with a friction clutch which are usually used in motor vehicles are simple and sturdy, but they have the disadvantage that an interruption in tractive power occurs during gear changes. Conventional automatic transmissions with torque converters avoid an interruption of tractive power during gear changes, but due to the torque converter they are less efficient than manual transmissions. Moreover, they are heavier and more cost-intensive.
In toothed gear transmissions having two frictionally engaged load shifting clutches and two partial transmissions disposed parallel to each other in the power flow, it is possible to change gears under load, which is done with an overlap control of the load shifting clutches. This makes it possible to achieve an inexpensive automatic transmission having good efficiency. Generally speaking, the even gear speeds are allocated to one partial transmission and the odd gear speeds are allocated to the other partial transmission, so that shifting can be done sequentially under load, although individual gear speeds cannot be skipped without further action, so-called double upshifting or double downshifting.
A generic toothed gear transmission is known from U.S. Pat. No. 4,658,663. In the toothed gear transmission, an input shaft is connected to an output shaft through a first and a second partial transmission, which are disposed parallel to each other in the power flow and which both have a frictionally engaged load shifting clutch. The load shifting clutches can be used to connect concentrically arranged intermediate shafts of the partial transmissions with the input shaft. The intermediate shafts can be effectively connected with the output shaft through transmission constants, through a joint countershaft disposed parallel to the input shaft, and through toothed gear pairs. The toothed gear pairs for the forward gear speeds I, III, and IV, as well as the toothed gear pair for the reverse gear speed, belong to the first partial transmission. The fourth gear speed is designed as a direct gear speed and can be selected by engaging a shifting component between the output shaft and the intermediate shaft of the first partial transmission.
The second partial transmission comprises the toothed gear pairs of forward gear speeds II and V, as well as an additional toothed gear pair, whose gear ratio is equal to that of the third gear speed, whose toothed gear pair belongs to the first partial transmission. In the first partial transmission, a toothed gear clutch for the third gear speed and the direct clutch for the fourth gear speed are combined into a change clutch having a joint synchronizer sleeve, which can be coupled with an idler gear of the toothed gear pair for the third gear speed or with the output shaft.
The load shifting clutches of the partial transmissions are alternately allocated to the gear speeds. The first load shifting clutch of the first partial transmission is always used to start from the first gear speed, which causes greater wear to occur on it than on the second load shifting clutch.